1. Field of the Invention
The present invention relates to an organopolysiloxane composition for molding purposes used for producing a molding matrix, which upon curing displays superior mold releasability relative to casting resin materials such as urethane resins, epoxy resins, dicyclopentadiene resins and polyester resins.
2. Description of the Prior Art
The production from an original mold of a silicone rubber matrix from an organopolysiloxane composition, and the subsequent injection of a material such as a urethane resin, an epoxy resin, a dicyclopentadiene resin or a polyester resin into this matrix to form a resin molded replica product is a well-known technique. In recent years, resin molded products produced in this manner have been supplied for use in automobile components and household electrical components, and the characteristics of such resin molded products are being given serious consideration. As a result, the improvements in the characteristics of the casting resins have been quite dramatic, although unfortunately these improvements have resulted in a deterioration in the durability of the silicone rubber matrix, and the number of replica products that can be produced from a single matrix has decreased. Consequently, improvements in durability of the mold releasability (hereinafter, xe2x80x9cmold release durabilityxe2x80x9d) or mold durability of the silicone rubber matrix with respect to these resins have been keenly sought.
An improvement in mold release durability upon addition of an alkali metal hydroxide to a silicone composition is disclosed in (Japanese) Laid-open publication (kokai) No. 4-216864 corresponding to U.S. Pat. No. 5,288,795, whereas in (Japanese) Laid-open publication No. 5-279571, an improvement in mold releasability is disclosed for compositions incorporating a compound selected from the group consisting of an organotin compound, an organotitanium compound and an imidazole derivative. A technique for improving polyester mold durability using a radical scavenger is disclosed in (Japanese) Laid-open publication No. 11-158385 corresponding to U.S. Pat. No. 6,251,327 and EP 0 905 196 A2. However, even with these modifications, the mold durability is still not entirely satisfactory. A composition using a polyfunctional cross-linking agent and a difunctional cross-linking agent is also disclosed in (Japanese) Laid-open publication No. 11-158385. However, the proportion of the difunctional cross-linking agent within the combined total of the cross-linking agents cannot be ascertained from the content of the above publication, and there is no indication that a combination of polyfunctional and difunctional chain extending agents produces superior mold release durability.
An object of the present invention is to provide an organopolysiloxane composition for molding purposes which displays superior mold releasability relative to materials such as urethane resins, epoxy resins, dicyclopentadiene resins and polyester resins.
The inventors of the present invention discovered that by combining a polyfunctional cross-linking agent and a difunctional cross-linking agent as a chain lengthening agent within an organopolysiloxane composition, and moreover by ensuring that the number of SiH groups within the difunctional cross-linking agent accounted for 20 to 70 mol % of the total number of SiH groups within the combined cross-linking agent, a composition could be produced which conformed to the above object, and as a result were able to complete the present invention.
In other words, the present invention provides an organopolysiloxane composition for molding purposes comprising
(A) an organopolysiloxane with at least two alkenyl groups bonded to silicon atoms in a single molecule, having a viscosity at 25xc2x0 C. of 0.05 to 100 Paxc2x7s,
(B) a straight chain organopolysiloxane with a hydrogen atom bonded to a silicon atom at both terminals and with no aliphatic unsaturated bonds within the molecule, having a viscosity at 25xc2x0 C. of 0.001 to 1.0 Paxc2x7s,
(C) an organohydrogenpolysiloxane with at least three hydrogen atoms bonded to silicon atoms within a single molecule and comprising a RHSiO unit and a R2XSiO1/2 unit (wherein R is an unsubstituted or a substituted monovalent hydrocarbon group with no alkenyl groups, and X represents a hydrogen atom or a group represented by R as defined above) within the molecule as essential components, having a viscosity at 25xc2x0 C. of 0.001 to 1.0 Paxc2x7s,
(D) an effective quantity of a hydrosilylation reaction catalyst,
(E) no more than 50 parts by weight of a finely powdered silica with a specific surface area of at least 50 m2/g per 100 parts by weight of the constituent (A), and
(F) 0 to 20 parts by weight of a non-functional organopolysiloxane having a viscosity at 25xc2x0 C. of 0.01 to 500 Paxc2x7s per 100 parts by weight of the constituent (A), wherein
the total number of hydrogen atoms bonded to silicon atoms within the aforementioned constituent (B) and constituent (C) is in a range of 1 to 5 atoms per alkenyl group within the aforementioned constituent (A), and the number of hydrogen atoms bonded to silicon atoms within the constituent (B) accounts for 20 to 70 mol % of the combined number of hydrogen atoms bonded to silicon atoms within the constituent (B) and the constituent (C).
An organopolysiloxane composition for molding purposes according to the present invention displays superior mold releasability relative to materials such as urethane resins, epoxy resins, dicyclopentadiene resins and polyester resins, and moreover also displays superior elongation at shearing and tear strength and can be suitably used as a highly durable mold composition.